1. Field of the Invention
The invention generally relates to a vacuum gauge. More particularly, the invention relates to a vacuum gauge that utilizes a piezoelectric material to convert a mechanical force to electrical charges according to the direct piezoelectric effect.
2. Description of the Prior Art
The conventional spinning rotor viscosity gauge comprises a sensor, two vertical stability control circular coils, a steel pipe and a steel ball. The steel ball is placed inside the steel pipe and one end of the steel pipe is sealed. The steel pipe can not be magnetized. In use, the steel ball is suspended between two permanent magnets of the sensor. The two vertical stability control circular coils are provided between the two permanent magnets to stabilize the steel ball in the vertical direction. In addition, two additional acceleration circular coils are provided to accelerate the steel ball. When the steel ball is accelerated to 400 RPS, these acceleration circular coils are turned off. The ball will slow down gradually due to the viscosity of the remaining air. The change in the rotational speed of the ball is then used to calculate the numerical value of the vacuum or air pressure. However, such spinning rotor viscosity gauge is too bulky and structurally complicated. There is a need for a vacuum gauge that is smaller, highly sensitive, highly accurate and of lower production cost and has a wider range of measurements.
In 1961, the Becker's research team developed a thin plate type vacuum gauge. A thin plate is placed in a magnetic field generated by a permanent magnet. Then, a motion signal is fed to the thin plate to make it vibrate. Because such vibration would cause a change in the magnetic field, an induced current is generated in the thin plate. In addition, the damping force of the ambient air or gas would reduce the amplitude of such vibration and hence the induced current would be reduced too. The change in the induced current is used to measure the numerical value of the air pressure. However, such thin plate type vacuum gauge is also structurally complicated and more expensive.
In 2007, K. Yohei et al developed a quarts friction gauge. Such quarts friction gauge is structurally simpler than other types of viscosity gauge because it uses the difference in impedance to measure the air pressure (the pressure varies with impedance exponentially). However, because quarts has a low efficiency in the conversion of mechanical force to electricity, such gauge can not be used to measure lower pressures near true vacuum.
In the viscosity vacuum gauge, the viscosity of the remaining air or gas is utilized to measure the pressure. Damping forces may be calculated by the following formula. With regard to the free damping of a flexible object, the decay rate of the vibration amplitude of the flexible object has a high correlation with the damping force of environment.
As the damping force increases, the amplitude would decrease. When an external simple harmonic force exerts on a system, the following may be used to calculate the amplification rate of the system:
      X          δ      st        =      1                                        (                          1              -                              r                2                                      )                    2                +                              (                          2              ⁢              ζ              ⁢                                                          ⁢              r                        )                    2                    where X is the amplitude; δst is the static replacement; r is the ratio of the driving frequency and the natural frequency; ζ is the ratio of the damping forces.
If ζ<0.05 and r=1 the driving frequency is equal to the natural frequency), the amplification rate of the system will be:
      X          δ      st        =            1              2        ⁢        ζ              =    Q  Where Q is defined as “quality factor”, and there would be two frequencies, R1 and R2 correspond to the amplitude
      Q          2        ,R1 and R2 are called “half efficiency points” and the difference between these two points is called bandwidth. Therefore, we can obtain:
  ζ  ≈                    R        2            -              R        1                    2      ⁢              ω        n            where ωn is the natural frequency of the system.